solubility

Question 1

define dissolution

Answer 1

ions on the surface of the solid solute are continually passing into the solution as hydrated ions

Question 2

define precipitation

Answer 2

hydrated ions from the solution are deposited on the surfaces of the solid sollute

Question 3

when is dynamic equilibrium reached?

Answer 3

when rate of dissolution and rate of precipitation become equal with concentrations of aqueous ions remaining constant

Question 4

what is this topic primarily looking at?

Answer 4

sparingly soluble ionic compounds which exist in dynamic equilibrium with their constituent aqueous ions at saturation

Question 5

define solubility

Answer 5

the maximum mass(in g) or amount (in mol) of the solute that can be dissolved per dm^3 of solvent to produce a saturated solution at a given temperature

solubility of a sparingly soluble ioni compound, at a given temperature, is determined experimentally.

Question 6

define solubility product(Ksp)

Answer 6

an equilibrium constant which is the product of the molar concentrations of the dissolved/dissociated ions(each raised to its appropriate power) in a saturated solution of the ionic compound at a given temperature.

numerical value of Ksp is always very small, rarely higher than 10^-4.

Question 7

How and why does Ksp value change with temperature?

Answer 7

1. value of Ksp changes only with temperature
2. when temperature increases, Ksp usually increases, because dissolution of ionic compound is usually an endothermic process = absorb heats by LCP, favours forward reaction

Question 8

what is the method for calculating Ksp for sparingly soluble ionic compounds given the solubilities.

Answer 8

1. write the partial dissociation equation for sparingly soluble ionic compounds
2. write solubility product expression, Ksp
3. calculate equilibrium concentrations for each ion from the solubility of the compound using mole ratio
4. substitute equilibrium concentrations into solubility product expression and solve

Question 9

what is the method for calculating solubility for sparingly soluble ionic compounds from Ksp.

Answer 9

1. let unknown solubility of ionic compound be X mol dm^-3.
2. write the partial dissociation equation for sparingly soluble ionic compounds
3. write the solubility product expression, Ksp
4. determine equilibrium concentrations for each ion from the solubility of the compound using mole ratio, in terms of X.
5. substitute equilibrium concentrations into solubility product expression and solve.

Question 10

how to compare solubility of ionic compounds?

Answer 10

If different formula type, must calculate/use solubility value

If same formula type, can use either Ksp or solubility values

Question 11

describe the common ion effect.

Answer 11

write down all necessaty half equations first

by LCP, the presence of a greater [X(aq)] causes poe to shift left to reduce concentration of common ion, X-, further surpressing dissociation of MX and hence reducing solubility of MX

this results in MX being precipitated out.

Question 12

what to take note of when doing calculations for questions involving common ion effect

Answer 12

in the ICE table,

0.20 + X is essentially = 0.20 as X is negligible.

Question 13

describe how formation of complex ion increases solubility

Answer 13

1. Ag+(aq) + Cl-(aq) = AgCl(s)
2. Ag+(aq) + 2NH3(aq) = [Ag(NH3)2]+(aq)

when aqueous ammonia is added to solution containing precipitate AgCl, it combines with the Ag+(aq) ions in the solution to form the diammine silver(I) complex ion, [Ag(NH3)2]+, which is soluble in aqueous solution.

this decreases [Ag+(aq)], resulting in a corresponding decrease in ionic product to below its Ksp. Hence, precipitate of AgCl in the solution dissolves to form a colourless solution.

Question 14

how does pH of solution affects solubility of an ionic compound.

Answer 14

e.g.
Addition of [H+(aq)]/removal of [OH-(aq)] will cause poe to shift right to to increase [OH-(aq)] by LCP, resulting in solubility of Pb(OH)2 to increase.

Question 15

define ionic product

Answer 15

it is the product of molar concentrations of the constituent ions in thr solution at that instant raised to the appropriate powers at a given temperature.

refers to the concentrations in a solution at any one time
solution may or may not be saturated
calculated using the instantaneous concentrations of ions upon mixing

Question 16

what happens when Ksp value is smaller, greater or equal to ionic product?

Answer 16

ionic product > Ksp: beyond saturation point, precipitation will occur

ionic product = Ksp: at saturation point, precipitation will not occur

ionic product < Ksp: below saturation point, precipitation does not occur